The optokinetic reflex (OKR) is a second mechanism by which the CNS stabilizes the visual scene on the retina, and it often works in conjunction with the VOR. Whereas the VOR is activated only by head motion, the OKR is activated by movement of the visual scene, whether caused by motion of the scene itself or by head motion. Specifically, the sensory stimulus for this reflex is slip of the visual scene on the retina as detected by motion-sensitive retinal ganglion cells. An example of the former occurs when you are sitting in a train and a train on the adjacent track begins moving: Your eyes rotate to keep the image of the neighboring car stable. This often leads to a sensation that you are moving (this is not entirely surprising because OKR circuits feed into the same circuits as used by the vestibular system).
The OKR can work in conjunction with the VOR to stabilize the visual image and is particularly important for maintaining a stable image when head movements are slow because the VOR works poorly in these conditions.
In animals whose eyes have a fovea, it becomes particularly advantageous to be able to move the eye in relation to the world (i.e., the main visual scene) so that objects of importance can be focused onto the fovea and scrutinized with this high-resolution part of the retina. Two classes of eye movement underlie this ability: saccadic and smooth pursuit. Very rapid discrete movements that bring a particular region of the visual world onto the fovea are called saccades. For example, to read this sentence, you are making a series of saccades to bring successive words onto your fovea to be read. However, even in animals that lack a fovea, the eyes make saccades, and thus saccades may also be used to rapidly scan the visual environment.
Saccades are extremely rapid eye movements. In humans, eye velocity during a saccade can reach 800 degrees/second, in comparison with movement velocity of less than 10 degrees/second generated in response to typical VOR and
CHAPTER 9 Organization of Motor Function
OKR stimuli (velocities of up to ≈120 degrees/second can be produced by OKR stimuli in humans; however, they are still much slower than the maximal saccade velocities). Saccades can be made voluntarily or reflexively. Moreover, although they are usually made in response to visual targets, they can also be made toward auditory or other sensory cues, in the dark, or toward memorized targets.
Interestingly, visual processing appears to be suppressed just before and during saccades, particularly in the magnocellular visual pathway that is concerned with visual motion. This phenomenon is known as saccadic suppression and may function to prevent sensations of sudden, rapid movement of the visual world that would result during a saccade in the absence of such suppression. The mechanisms underlying saccadic suppression are not fully known, but in areas of the cortex related to visual processing, the responsiveness of the cells to visual stimuli is reduced and altered during saccades.
Once a saccade has brought a moving object of interest onto the fovea, the smooth pursuit system allows the person to keep it stable on the fovea despite its continued motion. This ability appears to be limited to primates and allows prolonged continuous observation of a moving object. Note that in some respects, smooth pursuit might seem similar to the OKR; in fact, there may not be an absolute difference because as the target size grows, the distinction between target and background is lost; however, for small moving targets, smooth pursuit requires suppression of the OKR. You can see the effect of this suppression by moving your finger back and forth in front of this text while tracking it with your eyes. Your finger will be in focus, but the words on this page will be part of the background scene and will become illegible as they slip along your retina.